Gravitational lenses reveal the universe’s brightest galaxies

Hubble images of the brightest star-forming galaxies in the distant universe have revealed these galaxies are all affected by a phenomenon called gravitational lensing.

NASA, ESA, and J. Lowenthal (Smith College)

The term “gravitational lensing” has become pretty commonplace. This effect, which occurs when light from a background object, such as a galaxy, is magnified and brightened when it encounters a massive gravitational field, say from a galaxy cluster, on its way to Earth. Gravitational lensing can make otherwise impossible-to-see objects visible, and offers a window into the very distant universe. It also turns out, gravitational lensing is responsible for many, if not all, of the brightest infrared galaxies we see in the sky.

James Lowenthal of Smith College made the announcement Tuesday afternoon at a press conference during the 230th Meeting of the American Astronomical Society, which is taking place in Austin, Texas. Lowenthal and his collaborators are interested in studying galaxies called ultra-luminous infrared galaxies, or ULIRGS, which are undergoing huge booms of star formation in the faraway universe. However, star formation produces dust as a natural result; because these galaxies are dusty, much of their optical light is hidden and reprocessed by the dust, which re-emits the light at longer wavelengths: the infrared. Understanding why these galaxies are undergoing such intense star formation is vital to creating a more complete picture of galaxy evolution over time.

Lowenthal’s group began with data taken by the Planck satellite, which was launched to map the cosmic microwave background left over from the Big Bang. But because the satellite observed the sky in infrared and submillimeter wavelengths, it was also able to spot bright infrared galaxies. From this data, Lowenthal’s team assembled a sample of 31 of the brightest sources – some of “the very brightest infrared galaxies in the universe,” Lowenthal said during the press conference. These sources are star-forming galaxies that existed between 8 and 11.5 billion years ago, churning out stars at a rate 1,000 or more times that of the Milky Way’s current star formation rate (about one solar mass per year). In fact, they’re so active that “they’re not just ULIRGS, they’re 10 or 100 times the ULIRG threshold,” said Lowenthal. “They really are the most luminous objects that we know of.”

They team followed up their sample by looking at data taken with the ESA's Herschel Space Observatory and the Very Large Array. Finally, they used the Large Millimeter Telescope to observe their galaxy sample to measure their distances.

But because observing in longer wavelengths reduces the resolution, or sharpness, of the data, the team was still missing information about the nature of these galaxies. In particular, it was still difficult to tell why they were forming stars at such high rates. So they next turned to the Hubble Space Telescope (HST); while ULIRG galaxies don’t normally put out a lot of optical light because it’s obscured by dust, these galaxies are so extreme that they still emit enough for Hubble to pick it up.

Now, the first 11 of 31 have been imaged by HST, and the result is already astounding: These galaxies are all gravitationally lensed. “They knocked our socks off,” Lowenthal said. “This has been a treasure box, a jewel box of cool new images. And one after another, you see … gravitational lenses galore.”

What does that mean? These galaxies are all made brighter and bigger by the presence of galaxy clusters containing huge amounts of mass between the ULIRG and Earth. At least eight of the images show Einstein rings, an artifact of lensing that can smear the distant galaxy into a circular shape as a result of the viewing geometry. Lowenthal likened it to looking at a candle through a wine glass held longwise. If the glass is tilted just right, the image of the candle will smear out into a circle.

“We have added significantly to the total list of known gravitational lenses without even trying,” Lowenthal said. “We did not set out to find gravitational lenses. We set out to study distant, dusty starburst galaxies. But it turns out the brightest ones are all gravitationally lensed.”

These lensed images also show “dramatically more detail” than images captured with other instruments. And despite the distorted images created by the lenses, Lowenthal’s team can use these new, clearer images to reconstruct the galaxies – to, he said, “unscramble the true shape and nature of the background galaxies. And we can do it with better precision than we could before.”

This unprecedented detail will allow astronomers to peer deeper into the mechanisms responsible for these galaxies’ star formation on smaller scales within the galaxy itself, as small as 10 to 100 light-years across. Currently, there are two theories behind such huge bursts of star-forming activity in the distant universe: mergers between galaxies that excite material into forming stars, and cold gas flooding into galaxies from the intergalactic medium to feed star formation. In nearby galaxies, the former is responsible, but in these more distant galaxies, the question remains. The information needed to discern between the two ideas might be found inside these gravitationally lensed galaxies.

Lowenthal concluded the press conference by showing the attendees a sneak peek of the newest image, which he’d received while at the conference. And, just as the others in his sample: “It’s another one,” he said, as the image appeared on the screen to confirm it. “It’s another spectacular gravitational lens.”